The field of the invention is medical imaging and particularly, methods for reconstructing images from acquired image data.
In a computed tomography system, an x-ray source projects a fan-shaped beam which is collimated to lie within an x-y plane of a Cartesian coordinate system, termed the “image plane.” The x-ray beam passes through the object being imaged, such as a medical patient, and impinges upon an array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object and each detector produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all the detectors are acquired separately to produce what is called the “transmission profile,” “attenuation profile,” or “projection.”
The source and detector array in a conventional CT system are rotated on a gantry within the imaging plane and around the object so that the angle at which the x-ray beam intersects the object constantly changes. The transmission profile from the detector array at a given angle is referred to as a “view” and a “scan” of the object comprises a set of views made at different angular orientations during one revolution of the x-ray source and detector. In a 2D scan, data is processed to construct an image that corresponds to a two dimensional slice taken through the object. The prevailing method for reconstructing an image from 2D data is referred to in the art as the filtered backprojection technique. This image reconstruction process converts the attenuation measurements acquired during a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a display.
In general, cardiac CT imaging is a particularly demanding task. For example, sub-millimeter isotropic spatial resolution is necessary in order to visualize the small branches of the coronary arteries. Initially, computed tomography of the heart was performed by electron beam CT (“EBCT”) without contrast media to assess coronary calcifications. The lack of moving parts enables EBCT to achieve scan times of 50 milliseconds or less. While EBCT acquisition provides high temporal resolution, it suffers from low spatial resolution, for example, 1.2 millimeter in-plane resolution with 3 millimeter slice thickness. In recent years, tremendous improvements have been made in conventional rotating-gantry CT. In the state-of-the-art 64-slice, or 320-slice, multi-row detector CT (“MDCT”), 0.6×0.6×0.6 mm3 isotropic spatial resolution is achievable. Moreover, a state-of-the-art CT gantry may revolve at 0.27 seconds per revolution for single source-detector system, which allows for cardiac imaging with 150 millisecond temporal resolution.
According to standard image reconstruction theories, in order to reconstruct an image without aliasing artifacts, the sampling rate employed to acquire image data must satisfy the so-called Nyquist criterion, which is set forth in the Nyquist-Shannon sampling theorem. Moreover, in standard image reconstruction theories, no specific prior information about the image is needed. On the other hand, when some prior information about the desired image is available and appropriately incorporated into the image reconstruction procedure, an image can be accurately reconstructed even if the Nyquist criterion is violated. For example, if one knows a desired image is circularly symmetric and spatially uniform, only one view of parallel-beam projections (i.e., one projection view) is needed to accurately reconstruct the linear attenuation coefficient of the object. As another example, if one knows that a desired image consists of only a single point, then only two orthogonal projections that intersect at said point are needed to accurately reconstruct the image point. Thus, if prior information is known about the desired image, such as if the desired image is a set of sparsely distributed points, it can be reconstructed from a set of data that was acquired in a manner that does not satisfy the Nyquist criterion. Put more generally, knowledge about the sparsity of the desired image can be employed to relax the Nyquist criterion; however, it is a highly nontrivial task to generalize these arguments to formulate a rigorous image reconstruction theory.
The Nyquist criterion serves as one of the paramount foundations of the field of information science. However, it also plays a pivotal role in modern medical imaging modalities such as magnetic resonance imaging (“MRI”) and x-ray computed tomography (“CT”). When the number of data samples acquired by an imaging system is less than the requirement imposed by the Nyquist criterion, artifacts appear in the reconstructed images. In general, such image artifacts include aliasing and streaking artifacts. In practice, the Nyquist criterion is often violated, whether intentionally or through unavoidable circumstances. For example, in order to shorten the data acquisition time in a time-resolved MR angiography study, undersampled projection reconstruction, or radial, acquisition methods are often intentionally introduced.
The risks associated with exposure to the ionizing radiation used in medical imaging, including x-ray computed tomography (“CT”) and nuclear myocardial perfusion imaging (“MPI”), have increasingly become a great concern in recent years as the number CT and nuclear MPI studies has dramatically increased. The reported effective radiation dose from a cardiac CT angiography session is approximately 5-20 millisievert (“mSv”) for male patients and even higher for female patients. This dose is in addition to the smaller radiation dose from the calcium scoring CT scan that is routinely performed prior to intravenous contrast injection. To perform CT-MPI as part of a comprehensive cardiac CT study would require acquiring images over the same region of the heart approximately 20-30 times, resulting in an increase in radiation dose of approximately twenty- to thirty-fold, which is an unacceptable level of radiation exposure.
Additional major limitations exist with current cardiovascular CT imaging. In particular, current cardiac CT imaging methods suffer from inadequate temporal resolution to provide high quality cardiac images in all patient subsets. Thus, improving temporal resolution with CT cardiovascular imaging enables more accurate diagnoses and potentially safer, more effective therapeutic interventions.
Despite the short gantry rotation time of current state-of-the-art multi-detector CT (“MDCT”) imaging systems, the temporal aperture is inadequate to accurately measure global function (ejection fraction), assess wall motion abnormalities, or freeze valve motion to assess valvular abnormalities. In general, the temporal aperture should be no longer than 40-50 milliseconds in order to accurately assess global function and local wall motion abnormalities. In addition, despite current short gantry rotation times, pharmacological intervention (e.g., the administration of beta blockers) is often needed to slow the heart rate sufficiently in order to acquire images free from motion artifacts. This presents a limitation in the use of such a method since beta blockers are contraindicated in patients with impaired heart conduction and pulmonary disease, such as asthma. These are additional barriers for wider implementation of cardiac CT angiography.
In MDCT, the temporal resolution is primarily limited by the gantry rotation speed. In order to accurately reconstruct an image, the projection data is typically acquired over an angular range of 180 degrees, and greater. This angular range covers about two-thirds of a complete circle. After the incorporation of an appropriate weighting function in the employed image reconstruction algorithm, the typical temporal aperture of MDCT is limited to 50 percent of the gantry rotation time for a complete rotation. The temporal resolution and corresponding gantry rotation speed for the state-of-the-art MDCT scanners are summarized below in Table 1.
TABLE 1Temporal Resolution ChartGantry Rotation SpeedX-ray Source TypeTemporal Resolution350 msSingle175 ms350 msDual 87 ms270 msSingle135 ms
In cardiac MDCT imaging, to achieve better than 20 milliseconds temporal resolution, a gantry rotation period on the order of 50 milliseconds is required, which is currently infeasible due to mechanical limitations of CT imaging systems. Since the gantry speed is limited, segmented reconstruction has been investigated to improve temporal resolution. In segmented reconstruction, the data coverage required for a single reconstruction is filled with projection data selected from different heartbeats at the same cardiac phase. Using segmented reconstruction, the temporal resolution may be improved.
In the best case scenario, the temporal resolution can be improved by a factor of N (where N is the number of sectors utilized). However, segmented reconstruction is highly dependent on the consistency of the heart motion from one cycle to the next. It is noted that the gantry rotates several times during a single heart beat. When multiple heart beats are needed for segmented reconstruction, the projection data are distributed over many gantry rotations. Thus, due to the possible synchronization between the gantry rotation and the heart beat, the segments selected from different heartbeats often do not combine to produce a short-scan dataset. Therefore, the union of the segmented data sets does not provide a complete projection data set for an accurate image reconstruction. As a result, the promise of temporal resolution improvement in segmented reconstruction is not reliably achieved and, thus, the segmented reconstruction method is rarely utilized in clinical practice.
It would therefore be desirable to provide a method for reconstructing an image of a dynamic subject, such as the beating heart, with a temporal resolution on the order of 40 milliseconds the produces images substantially free of undesired image artifacts.